High power transmission line filters



Nov 4, 1958 J, VQGELMAN 2,859,418

HIGH POWER TRANSMISSION LINE FILTERS Filed June '21 1955 INVENTOR.ddS'P/l bi. V06 (Mi/V 2,859,418 HEGH POWER TRANSMESSION LINE FILTERSJoseph H. Vogelinan, Rome, N. Y.

Application dune 21, 1955, Serial No. 517,102

8 Claims. (Cl. 333-73) (Granted under Title 35, U. S. Code (1952), see.266) The invention described herein may be manufactured and used by orfor the United States Government for governmental purposes withoutpayment to me of any royalty thereon. g

This invention relates to high power waveguide filters capable ofhandling powers equal to that of the waveguide or coaxial transmissionline with which the filter is used.

There are numerous devices in the prior art utilizing constrictedwaveguide filtering techniques which are unsuitable for high powerapplications by their general nature, and in addition are either of thehigh pass filter type which relies upon the cutofi. frequency of awaveguide or are of the resonant cavity variety.

Tapered transmission lines, both of the waveguide and coaxial type havelong been known and the characteristics have been determined and havebeen fully set forth at pages 2954 of the Wave Guide Handbook of theRadiation Laboratory Series of the Massachusetts Institute ofTechnology, first edition, published by the McGraw-Hill Book Company,Inc.

The filter of this invention is ofthe high power band pass type andrelies solely upon impedance discontinuities in a transmission line toachieve filtering action. The filter consists of a series of symmetricaldiscontinuities of size distributed along a transmission line which maybe of the waveguide or coaxial type or of any other type derivabletherefrom. These discontinuities consist of changing from a uniformtransmission line to a tapered section and from a taper of one angle toa taper of another angle. The characteristic of the impedance at thejunctionof a rectangular and a tapered or radial guide together with theequivalent circuit is set forth at pages 322'and323 of the Wave GuideHandbook above noted. These variations are limited to changes whichaffect the impedance of the transmission line while keeping theWaveguide wavelength constant. Furthermore, constrictions of thestandard transmission line can not be used for high power applicationsso that all of the tapers are such that the minimum diameter of anyfilter section is that of the associated transmission line. Theimpedance mismatch properties resulting from such tapers are used toobtain the desired filtering action. This type of filter considerablyimproves the operation of high pulsed power multi-channel systems usinga common antenna system.

It is therefore an object of this invention to provide a filter thepower handling capability of which is equal to that of the transmissionline into which it is inserted and which will have a zero insertion lossat the design frequency when the parameters are properly selected.

It is a further object of this invention to provide for the synthesis ofa Wide variety of filter characteristics by providing a suflicientnumber of variable parameters to achieve the desired results and toprovide a filter design which can be readily scaled from one frequencyband to another and from one transmission line size to another.

These and other objects and advantages will be apparent to those skilledin the art from the following de- .ates atent ice .2 tailedspecificationand attached 'drawing forming apart thereof and wherein: I

Figure 1 is a schematic cross section of a filter inserted in, awaveguide transmission line.

Figure 2 is a schematiecrosssection of afilter inserted.

in. a coaxial transmissionline.

Figure 3 is an equivalent'circuit for-the filter'of Figures 1 and 2.

Turning now to the drawings it will be seen that the waveguide,whichxmaynhave any. suitable crosssection such: as rectangular orcylindrical, filter of Figure land the coaxial line filter of'Figure 2'both are shown as consisting of three sections. Section I consists of anexpand-' ing transmission line producing a discontinuity. at plane 1 asa result of the junction-eaused-by the expanding transmission line andtheuniform linesW or W respectively.

Thediscontinuity at plane lihas an impedance XI shown in the equivalentcircuit of: Figure. 3. which impedance is' determined by the angle. 0 or0 respectively. At plane 2 an impedance discontinuity results. from theabrupt change in going fromsection Ixtosection II and the magnitude ofthis discontinuity has an impedance X which is a function of the angle.0 uor 0 respectively. Section II of either the waveguide or coaxial typecan consist of a larger size uniform transmission line or an expandingor contracting transmission line propagating high order modes ofoperation'asmaybe desirable for the tion of lineof one dimension toanother section of line This, however, requires tapered of anotherdimension. sections several times the wavelength at the operatingfrequency in order to avoid impedance discontinuities. In

' the present filter, each tapered section is deliberately made of the.same order of magnitude. as. the wavelength at the center frequency ofthe pass band in: orderto introduce desired impedance discontinuities.

To obtain a filter of given characteristics itis necessary to, solve theequivalent transmission line circuit of Figure 3. For any. given.waveguide or transmission line within the frequency band of interesteach'of the discontinuities X through X, are experimentally measuredindividually as a function of the angles 0 through 0 respectively. Theimpedances and line lengths L through L, of Figure 3 are selected togive the desired resonances at the specified frequency; that is,terminating the terminals 7, 8 in the characteristic impedance Z shouldresult in an input impedance at terminals 5, 6 which is equal to Z Inpractice this is accomplished by utilizing standard and well-knownnetwork synthesis methods to obtain an equivalent circuit having thedesired frequency characteristics. From the equivalent circuit for thevarious frequencies in the band under consideration the appropriatediscontinuities and line lengths are determined. From the data givingthe discontinuity impedances resulting from variation of 0 the properarrangement and physical embodiment of discontinuities can then bedetermined.

An alternative method of synthesis of this type of filter would consistin determining the resonances and antiresonances desired of the filter.Then, by standard transmission line techniques, the input impedancesexpressed as a function of the discontinuities at the planes 1, 2, 3,and 4 and lengths L L and L could be determined. The values of thediscontinuities and the line lengths could In. the. coaxial type offilter shownthen be selected to give the desired resonances andantiresonances in the frequency region under consideration. For example:to obtain a filter resonant at f and antiresonant at and about fdetermine the. transmission line equation Z =F(X X X X4, 'L L L appro-'priate to the particular transmission line system and design frequency.This eq'uationis solved for values f f and to giverthe resonance andantiresonances desired and relationships are obtained between X, and LFrom the experimental .data describing variation of X as a function of 0it is then possible to obtain values of angles 6 and lengths L to give aphysically realizable structure.

, While a specific preferred embodiment of the invention has beendescribed for purposes of illustration it is under-' stood that theinvention is defined solely by the appended claims.

o What I claim is: i

1. A high power handling band pass filter comprising, a uniformtransmission line having a. first and second section, a plurality ofcontiguous tapered-line filter sections inserted in said uniform 'line'between said first and second sections, said tapered-line sectionsproducing a plurality of impedance discontinuities, each of said filtersections having a lengthwhich is of theLsame order to magnitude as theoperating wavelength at 'the center of the pass band and, having aminimum cross sectional dimension which is at least as great as theminimum cross sectional dimension of said uniform transmission line,said impedance discontinuities being a function of the'.angles formed'bythe junctions of said tapered sections.

2. Apparatus as in claim 1 .wherein said uniform transmission line isawaveguide and wherein the first of said plurality of tapered filtersections is an expanding waveguide section and the last of saidplurality of tapered filter sections is a contracting waveguide section.

3. Apparatus as in claim 1 wherein said uniform transmission line is acoaxial cable and wherein the first of said plurality of tapered filtersections is an expanding section of coaxial cable having an outer and aninner conductor both of 'Which have an expanding taper, and wherein thelast of said plurality of tapered filter sections is a contractingsection of coaxial cable having an outer and inner conductor both ofwhich have a contracting taper.

4. A high power band pass filter system comprising a first and secondsection of uniform transmission line, a series of radial sections oftransmission line connected between said first and second sections, eachof said radial sections having a length which is of the same order ofmagnitude as the operating wavelength of the system, each of said radialsections having a minimum cross-sectional dimension which is at least asgreat as the crosssectional dimension of said first and second sectionsof uniform transmission line.

5. For use in a high power transmission line having a first and a secondWave guide section of uniform cross section, a high power handling bandpass filter comprising a first expanding radial wave guide filtersection, a second contracting radial wave guide filter section, theminimum cross section of each of said radial wave guide filter sectionsbeing coextensive with the cross section of the respective wave guidetransmission line sections, an intermediate wave guide filter sectionconnecting said first and second radial wave guide'filter sections, eachof said filter sections having a length which is of the same order ofmagnitude as the operating wavelength of the center of the pass band,said radial wave guide filter sections producing an impedancediscontinuity at each junction.

guide filter section connecting said first and second radial Wave guidefilter sections, said intermediate filter section being a radial waveguide, each of said filter sections having'a length which is of the sameorder of magnitude as the operating wavelength of the center of the passband, said radial wave guide filter sections producing an impedancediscontinuity at each junction.

7. Apparatus as in claim 5 wherein the filter section between saidexpanding and contracting filter sections is enlarged to propagate highorder modes to produce the filtering effect.

8. Apparatus as in claim 5 wherein the intermediate filter section is auniform line section, the TE mode being excited therein.

References Cited in the file of this patent UNITED STATES PATENTS2,533,239 Gent et al. Dec. 12, 1950 2,737,630 Miller Mar. 6, 19562,738,468 Miller Mar. 13, 1956 2,738,469 Miller Mar. 13, 1956 2,747,184Kock May 22, 1956

